1. Field of the Invention
The present invention relates to a method and apparatus for image forming, and more particularly to a method and apparatus for image forming that is capable of performing an effective fixing process.
2. Description of the Related Art
Under an increasing demand for conservation of natural resources and saving energy in the scope of a global environment protection, considerable efforts in reducing consumption of electric power are made in the field of electrophotographic image forming apparatuses such as copying machines, facsimile machines, printers, plotters, and so on. Among various processes of image forming, a fixing process particularly consumes a great amount of electric power and a technique of a low temperature fixing is expedited in this field. To succeed in the low temperature fixing, it is necessarily needed to lower a softening or melting point of toner. A thermoplastic resin included in the toner has a character that lower the softening or melting point lower a melting viscosity. This character is based on a fact that the softening or melting point of a thermoplastic resin is determined by various factors such as molecular weight, distribution of molecular weight, the level of crystallization, the level of bridging, intermolecular force, and so forth. Therefore, in order to lower the softening or melting point of a thermoplastic resin without changing its structure, it is needed that the molecular weight or the level of bridging is reduced or that the distribution of molecular weight is narrowed. Since the distribution of molecular weight has a lower limitation which is determined by a storage limitation of the resin, it is narrowed when the molecular weight is reduced.
In general, when molecular weight is reduced, chains of molecules are shortened and the connections between the molecules are loosened. Therefore, the melting viscosity is lowered. Also, when the distribution of molecular weight is narrowed, the connections between the molecules are loosened and therefore the melting viscosity is lowered. Further, when the level of bridging between molecules is lowered, each molecule becomes easy to move and therefore the melting viscosity of the molecules is lowered.
For example, a published Japanese examined patent application No. 51-29825 (1976) describes a fixing method which performs a fixing process using toner that has a lowered melting viscosity, as described above, without causing an offset. The offset in the fixing process is a problematic phenomenon in which toner is undesirably deposited on a part of a fixing roller by loosing its character of cohesion when melted. The fixing of toner is performed when the toner is in a rubber state. That is, as a temperature rises, the toner resin begins to be softened and its viscosity is lowered. Then, the toner resin is brought to a state of rubber. As far as being in the rubber state, the toner resin maintains a relatively high cohesion and does not cause the offset problem.
A Japanese Patent, No. 2516886, describes an apparatus for heating an image using the above-mentioned technique. This apparatus includes a line-shaped heating member based on a heating member described in the above-mentioned published Japanese examined patent application, No. 51-29825 (1976), and is characterized by a feature in that the line-shaped heating member is energized with a pulse signal. This feature attempts to eliminate a residual heat needed for reduction of a standby time and to reduce emission of an extra amount of heat inside the apparatus.
The above-mentioned background techniques and apparatuses, however, may only be effective when the apparatus processes a small number of images or when the apparatus is almost out of busy state. When a large number of images are processed, the recording sheets take a great amount of heat. This causes a loss of a great amount of energy, regardless of whether a roller-shaped or line-shaped heating member is used.
However, in most cases, an image to be actually printed on a recording sheet has a substantial area in the range between 2% and 10% relative to a recording area in a recording sheet. This means that heat is taken also by a 90% to 98% area of a recording sheet without being used. For example, a text image that has lines of characters typically includes non-image spaces between the lines and the heat applied to these non-image spaces are not used.
Since the above-mentioned background techniques and apparatuses employ the toner having a relatively high softening or melting point, a partial application of heat to an image area in a recording sheet causes a fixing mechanism and a recording sheet to be regionally deformed. As a result, the recording sheet is transferred not in a properly straight manner or has wrinkles due to distortion.
The present application describes a novel fixing apparatus. In one example, a novel fixing apparatus includes a heater, an endless belt, a pressure roller, and a heater controller. The heater has a line shape orthogonal to a direction in which a recording sheet carrying an unfixed toner image formed with toner in accordance with image information is transferred. The endless belt is configured to be rotated with an inner surface thereof sliding over a surface of the heater. The pressure roller is arranged at a position opposite to the heater relative to the endless belt and is held for rotation in contact with the endless belt under pressure to form a nip therebetween. The heater controller is configured to energize the heater in accordance with the image information. In this novel fixing apparatus, when the recording sheet is brought to the nip with the unfixed toner image facing the endless belt, the pressure roller applies pressure to the recording sheet against the endless belt so that the unfixed toner image is fixed on the recording sheet with heat by the heater as the recording sheet is transferred by movement of the endless belt and the pressure roller.
The toner may include a resin as a main adhesive agent and has properties of a softening or melting point in a range between 50xc2x0 C. and 160xc2x0 C. and a viscosity in a range between 10 [c poise] and 1013 [c poise] under a temperature above the softening or melting point.
The heater may include at least two parallel heating elements, each of which has a line shape orthogonal to the direction in which the recording sheet is transferred.
The heater controller may alternately energize the above-mentioned at least two parallel heating elements with alternating pulses.
The above-mentioned at least two parallel heating elements may be distant from each other by 10 mm or less.
Each of the at least two parallel heating elements may have a width in a range between 0.01 mm and 5 mm.
The heater may include a plurality of heating elements arranged in line in a direction orthogonal to the direction in which the recording sheet is transferred.
Each of the plurality of heating elements may include a thermal head.
The heater controller may selectively energize the plurality of heating elements.
The above-mentioned fixing apparatus may further include a cooling mechanism configured to cool the toner image after the toner image is fixed with heat by the heater on the recording sheet.
The above-mentioned fixing apparatus may further include a guide roller arranged at a position downstream from the heater in the direction in which the recording sheet is transferred, the guide roller being configured to support the endless belt and to serve as a cooling mechanism configured to cool the toner image after the toner image is fixed with heat by the heater on the recording sheet.
The above-mentioned fixing apparatus may further include a mechanism configured to cause the endless belt to tightly hold the toner image and the recording sheet together until the toner image is fixed on the recording sheet after the toner image is subjected to the heat of the heater.
The heater controller may stop energizing the heater during a time when a non-image region between two adjacent toner image lines in the recording sheet is brought close to the heater.
The heater controller may energize the heater during a time when a region of the toner image in the recording sheet is brought close to the heater.
The heater controller may energize the heater with an electric power reduced by 5% or more during a time when a non-image region between two adjacent toner image lines in the recording sheet is brought close to the heater.
The present invention further provides a novel fixing method of image forming. In one example, a novel fixing method of image forming includes the steps of forming, proving, rotating, transferring and energizing. The forming step forms a nip between an endless belt and a pressure roller which are held for rotation in contact with each other under pressure. The proving step provides a heater at position inside the endless belt, in contact with the endless belt, and opposite to the pressure roller relative to the endless belt. The above-mentioned heater has a line shape orthogonal to a direction in which a recording sheet having an unfixed toner image formed with toner in accordance with image information is transferred. The rotating step rotates the endless belt and the pressure roller. In this case, the endless belt slides over a surface of the heater by rotation. The transferring step transfers the recording sheet to the nip. The recording sheet is in an orientation in which the toner image faces the endless belt. The energizing step energizes the heater in accordance with the image information when the toner image is brought to the heater.
The present invention further provides a novel image forming apparatus. In one example, a novel image forming apparatus includes an image forming mechanism, a heater, an endless belt, a pressure roller, and a heater controller. The image forming mechanism is configured to form a toner image with toner on a recording sheet in accordance with image information. The heater has a line shape orthogonal to a direction in which the recording sheet carrying an unfixed toner image formed by the image forming mechanism is transferred. The endless belt is configured to be rotated with an inner surface thereof sliding over a surface of the heater. The pressure roller is arranged at a position opposite to the heater relative to the endless belt and is held for rotation in contact with the endless belt under pressure to form a nip therebetween. The heater controller is configured to energize the heater in accordance with the image information. In the above-mentioned image forming apparatus, when the recording sheet is brought to the nip with the unfixed toner image facing the endless belt, the pressure roller applies pressure to the recording sheet against the endless belt so that the unfixed toner image is fixed on the recording sheet with heat by the heater as the recording sheet is transferred by movement of the endless belt and the pressure roller.